1. Field of the Invention
The present invention relates to a communication apparatus (communication system), transmitter and receiver, and communication method, more particularly relates to a digital communication apparatus (system) for multi-carrier modulation, a wireless transmitter (wireless transmitting apparatus) and a wireless receiver (wireless receiving apparatus) used in a digital communication apparatus (system), and a communication method of the same.
More specifically, the present invention relates to a signal component demultiplexing apparatus for demultiplexing a multi-carrier signal multiplexed by orthogonal frequency division multiplexing (OFDM) to a symbol series, a filter apparatus for extracting specific symbols from a multi-carrier signal, and a signal receiving apparatus having these signal component demultiplexing apparatus, filter apparatus, etc.
2. Description of the Related Art
As an example of a signal modulated by OFDM, one of a digital audio broadcasting (DAB) system will be described.
The DAB system is known as a high quality digital audio terrestrial broadcasting method enabling mobile reception developed by the EUREKA 147 project. Progress is being made in commercialization of digital satellite audio broadcasting using the DAB system for the satellite broadcasting.
As the modulation method used in such a digital communication system (apparatus), OFDM has been proposed due to its tolerance to multi-path fading, ghosts, etc.
OFDM is a multi-carrier modulation method usually using tens to hundreds of orthogonal carriers. Each carrier is modulated by a modulation method such as quadrature amplitude modulation (QAM) or phase shift keying (PSK).
In the DAB system etc., digital audio signals of multiple channels are transmitted by multi-carrier communication.
FIGS. 21A and 21B are views of an example of the configuration of a digital wireless communication system used in a DAB system or the like using OFDM as the multi-carrier modulation method. FIGS. 21A and 21B illustrate parts of the DAB system in a simple form.
In the following explanation, the DAB system will be illustrated and the explanation made focusing on multiplexing.
A wireless transmitting apparatus 10 of an OFDM type wireless communication system illustrated in FIG. 21A has an encoder circuit 11, a symbol mapping circuit 12, a multiplexer (signal multiplexing circuit) 13, a frequency interleave circuit 14, an inverse fast Fourier transform (IFFT) circuit 15, a wireless transmitter circuit 16, and an antenna 17.
An information bit stream is encoded, interleaved, and otherwise processed in the encoder circuit 11, then its bits are mapped to transmission symbols in the symbol mapping circuit 12. This work is separately carried out for every channel. In the example shown in FIG. 21A, for example, 64 ksps (symbols/sec) of symbols are created per channel.
These symbol streams are simply connected in series in the multiplexer 13 to form a multiplexed symbol stream. For example, if 18 channels of 64 ksps per channel are multiplexed, the transmission rate of the multiplexed symbol stream becomes 1152 ksps (=18xc3x9764 ksps).
The symbols of the multiplexed symbol stream are rearranged by frequency interleaving in the frequency interleave circuit 14. The symbols of each channel are dispersed by this work.
Next, the dispersed symbols of the symbol stream are arranged on the frequency axis, then the symbol expressions on the frequency axis are transformed to symbols on the time axis by the IFFT processing in the inverse fast Fourier transform (IFFT) circuit 15, which are then sent from the transmitter circuit 16 via the antenna 17 into the air.
An example of the symbol string comprised of the six carriers formed into a multi-carrier signal output from the transmitting apparatus 10 is illustrated in FIG. 22.
Up until now, specific symbols among a plurality of symbols (symbol series) formed into the above multi-carrier signal have not been solely extracted.
Therefore, we suppose the wireless signal receiver extracts the intended symbols or carrier components from the symbol series illustrated in FIG. 22 using an existing technique.
FIG. 23 is a view of a first method for demultiplexing a multi-carrier signal.
In this method, a plurality of band pass filters having frequency band characteristics of the corresponding carriers are provided. The corresponding symbols are extracted by these band pass filters. As such filters, use can be made of for example comb type filters.
However, such a method is unsuitable for demultiplexing symbols of a modulation method such as OFDM where the carriers are crammed together. Namely, with a modulation method using OFDM, a large number of carriers are crammed in a certain frequency band, therefore adjoining signal components cannot be sufficiently isolated. Accordingly, each band pass filter must have a sharp frequency characteristic in order to discriminate between carrier signals of adjoining frequencies.
For example, it is difficult to prepare various types of high precision filters such as comb type filters which have such sharp frequency characteristics. Further, this becomes considerably expensive in terms of price. Therefore, it is difficult to realize this.
FIG. 24 is a view of a second method of demultiplexing a multi-carrier signal.
In FIG. 24, a Fast Fourier transform (FFT) is applied to a signal received at a receiver circuit 22 in a Fast Fourier transform (FFT) circuit 23 to create a received symbol series arranged on the frequency axis. The symbol series is demultiplexed to separate symbols in a demultiplexer (signal demultiplexing apparatus) 29. Due to this, it is possible to select only specific symbols.
In this method, however, even when extracting specific symbols, the fast Fourier transform is applied to all symbols. Therefore, a complex FFT circuit 23 must be provided, so the hardware configuration becomes complex.
FIG. 25 is a schematic view of the configuration when extracting only carrier signal components at constant intervals. In FIG. 25, a plurality of band pass filters having a plurality of different band pass characteristics are provided. Signals limited in bands by the filters are added to each other at adder circuits 28A and 28B to obtain the intended signal. In this case as well, as the band pass filters, for example, comb type filters can be used.
However, in the same way as with the method of FIG. 23, since it is a multi-carrier method, this method also suffers from the disadvantages that carriers are crammed together, so the signal components cannot be sufficiently isolated. Also, it becomes difficult to prepare high precision filters having sharp frequency characteristics from the cost perspective etc.
FIG. 21B is a schematic view of the configuration of a wireless signal receiver in the DAB system illustrated in FIG. 21A.
A wireless receiving apparatus 20 of an OFDM wireless communication system 1 of FIG. 21B has an antenna 21, a receiver circuit 22, a Fast Fourier transform (FFT) circuit 23, a symbol selection circuit 24, a bit extraction circuit 25, and a decoding circuit 26.
By transforming the frequency of the signal of the intended frequency band received at the antenna 21 in the receiver circuit 22 and extracting only the baseband signal component, a baseband signal is obtained. The thus obtained baseband signal is expressed on the time axis of the signal with the information arranged on the frequency axis. Therefore, FFT processing is carried out in the FFT circuit 23 to extract subcarriers arranged on the frequency axis.
At this time, the symbols output by the FFT processing consist of the group of subcarriers of the signal bands received as a whole (for example, in the present example, containing 1152 ksps worth of information).
The symbol selection circuit 24 extracts the symbols from the group of subcarriers from the positions of the symbols of the intended channel arranged by the frequency interleaving at the transmission side illustrated in FIG. 21A. By this, the 64 kbps of information of the intended channel is extracted.
The received bit stream is extracted from among the symbol stream of the intended channel obtained in this way in the bit extraction circuit 25 to obtain the encoded bit stream, then this is decoded at the decoding circuit 26 to obtain the information bit stream of the intended channel.
Summarizing the disadvantages to be solved by the invention, in this way, in OFDM, multiplexing is carried out by allocating symbols of different channels to different subcarriers, but this means that the wireless receiving apparatus 20 receives a multiplexed signal of all channels transmitted and, further, that the FFT circuit 23 extracts the symbols of all of the channels, then the symbol selection circuit 24 selects the channel. Therefore, the FFT circuit 23 performs FFT processing entailing computations far exceeding the amount required for the originally required one channel""s worth of information.
Namely, this means that the FFT circuit 23 performs the FFT signal processing for even channels which the wireless receiving apparatus 20 does not desire, so there is a disadvantage in that the FFT circuit 23 becomes unnecessarily large in scale.
As a method of solving this disadvantage, the present inventors have proposed the invention disclosed in for example Japanese laid open patent No. 2000-332722 published on Nov. 30, 2000. In the invention disclosed in Japanese laid open patent No. 2000-332722, circuits for demultiplexing a symbol string for every alternate subcarrier from the symbol series are provided in multiple stages hierarchically by a branching method.
An object of the present invention is to overcome the disadvantage by a method different from that of the invention disclosed in Japanese laid open patent No. 2000-332722, and to further extract only one symbol with a high efficiency.
Another object of the present invention is to provide a signal component demultiplexing apparatus capable of demultiplexing a symbol series in a branching manner with a high efficiency.
Another object of the present invention is to provide a filter apparatus capable of extracting specific symbols from a symbol series with a high efficiency.
Still another object of the present invention is to provide a receiving apparatus having the signal component demultiplexing apparatus and/or filter circuit.
Still another object of the present invention is to provide a communication system having a receiving apparatus and a transmitting apparatus.
Still another object of the present invention is to provide a communication method for the receiving processing and the transmitting processing.
According to a first aspect of the present invention, there is provided a signal component demultiplexing apparatus for demultiplexing a certain group of signals from among a group of multi-carrier modulated signals (group of symbols), comprising branching circuits connected in stages and hierarchically by a branching method, each branching circuit including a symbol delaying means for delaying an input group of signals by N/2(m+1) symbols, a phase offset adjusting means for shifting the phase of the input group of signals by xe2x88x92xcfx80(k/2m) radians with a reference 0 Hz, an adding means for adding an output signal of the symbol delaying means and an output signal of the phase offset adjusting means to calculate one symbol string alternately positioned on a frequency axis in a multiplexed signal input to a signal selecting and outputting means, and a subtracting means for subtracting the output signal of the phase offset adjusting means from the output signal of the symbol delaying means to calculate the other symbol string alternately positioned on the frequency axis in the multiplexed signal input to the signal selecting and outputting means, wherein m is a parameter indicating the position of a stage of the branching circuit, N is the number of symbols existing within one modulation time, and k is a parameter indicating that a group of signals having a frequency offset of a subcarrier is input with a reference 0 Hz.
According to a second aspect of the present invention, there is provided a receiving apparatus, used in multiplex communication based on multi-carrier modulation where subcarriers of a plurality of channels are cyclically arranged, comprising the above signal component demultiplexing apparatus.
The receiving apparatus has a receiving means for receiving a group of signals; a signal component demultiplexing apparatus comprising branching circuits connected in stages and hierarchically by a branching method, each branching circuit provided with a symbol delaying means for delaying an input group of signals by N/2(m+1) symbols, a phase offset adjusting means for shifting the phase of the input group of signals by xe2x88x92xcfx80(k/2m) radians with a reference 0 Hz, an adding means for adding an output signal of the symbol delaying means and an output signal of the phase offset adjusting means to calculate one symbol string alternately positioned on a frequency axis in a multiplexed signal input to a signal selecting and outputting means, and a subtracting means for subtracting the output signal of the phase offset adjusting means from the output signal of the symbol delaying means to calculate the other symbol string alternately positioned on the frequency axis in the multiplexed signal input to the signal selecting and outputting means; an orthogonal transforming means for orthogonally transforming the group of signals demultiplexed at the signal component demultiplexing apparatus; and a decoding means for decoding the orthogonally transformed information.
According to a third aspect of the present invention, there is provided a communication apparatus having a transmitting apparatus and the above receiving apparatus in multiplex communication based on multi-carrier modulation where subcarriers of a plurality of channels are cyclically arranged.
The transmitting apparatus of the communication apparatus has an encoding means for independently encoding information of a plurality of channels, a signal point arranging means for arranging signal points by modulating the encoded information based on a predetermined modulation method, a signal multiplexing means for multiplexing the plurality of signal point-arranged signals cyclically on a time axis, an inverse orthogonal transforming means for inversely orthogonally transforming the multiplexed signal, and a transmitting means for transmitting the orthogonally transformed information.
The receiving apparatus of the communication apparatus has the same components as the above receiving apparatus, that is, a receiving means for receiving the transmitted group of signals, a signal component demultiplexing means for selecting and demultiplexing the received group of signals, an orthogonal transforming means for orthogonally transforming the selected and demultiplexed signal, and a decoding means for decoding the orthogonally transformed information.
The signal component demultiplexing means has the above configuration.
Preferably, the signal multiplexing means in the transmitting apparatus multiplexes the plurality of signal point-arranged signals while shifting the frequency for every channel at predetermined subcarriers.
More preferably, the modulation method in the signal point arranging means in the transmitting apparatus uses orthogonal frequency division multiplexing (OFDM).
Still more preferably, the inverse orthogonal transform processing means in the transmitting apparatus performs inverse Fourier transform processing, and the orthogonal transform processing means in the receiver performs Fourier transform processing.
According to a fourth aspect of the present invention, there is provided a communication apparatus used in multiplex communication based on multi-carrier modulation where subcarriers of a plurality of channels are cyclically arranged, comprising a receiving means for receiving a group of signals; a signal component demultiplexing apparatus comprising branching circuits connected in stages and hierarchically by a branching method, each branching circuit provided with a symbol delaying means for delaying an input group of signals by N/2(m+1) symbols, a phase offset adjusting means for shifting the phase of the input group of signals by xe2x88x92xcfx80(k/2m) radians with a reference 0 Hz, an adding means for adding an output signal of the symbol delaying means and an output signal of the phase offset adjusting means to calculate one symbol string alternately positioned on a frequency axis in a multiplexed signal input to a signal selecting and outputting means, and a subtracting means for subtracting the output signal of the phase offset adjusting means from the output signal of the symbol delaying means to calculate the other symbol string alternately positioned on the frequency axis in the multiplexed signal input to the signal selecting and outputting means; a signal selecting means for selecting and outputting at least one group of symbols of predetermined subcarriers from among the symbol strings demultiplexed at the signal component demultiplexing apparatus; a frequency offset compensating means for compensating for frequency offset of at least one group of symbols selected and output by the signal selecting means; two orthogonal transforming means for orthogonally transforming output signals of the frequency offset compensating means; and a decoding means for decoding the orthogonally transformed signal.
Preferably, the frequency offset compensating means has a frequency offset compensation signal generating means for outputting a complex sine wave signal for the frequency offset compensation, a multiplying means for multiplying the group of signals and the complex sine wave signal output from the frequency offset compensation signal generating means, and a rearranging means for rearranging symbols as the result of multiplication in the multiplying means along a frequency axis.
According to a fifth aspect of the present invention, there is provided a communication apparatus having a transmitting apparatus and a receiving apparatus of the fourth aspect of the invention used in multiplex communication based on multi-carrier modulation where subcarriers of a plurality of channels are cyclically arranged.
According to a sixth aspect of the present invention, there is provided a receiving apparatus used in multiplex communication based on multi-carrier modulation where subcarriers of a plurality of channels are cyclically arranged, provided with a receiving means for receiving a group of signals; a signal component demultiplexing apparatus for demultiplexing the received group of signals configured by branching circuits connected in stages and hierarchically by a branching method, each branching circuit including a symbol delaying means for delaying an input group of signals by N/2(m+1) symbols, a phase offset adjusting means for shifting the phase of the input group of signals by xe2x88x92xcfx80(k/2m) radians, an adding means for adding an output signal of the symbol delaying means and an output signal of the phase offset adjusting means to calculate one symbol string alternately positioned on a frequency axis in a multiplexed signal input to a signal selecting and outputting means, and a subtracting means for subtracting the output signal of the phase offset adjusting means from the output signal of the symbol delaying means to calculate the other symbol string alternately positioned on the frequency axis in the multiplexed signal input to the signal selecting and outputting means; a frequency offset compensating means for compensating for frequency offset of at least one group of symbols selected and output by the signal selecting means; two orthogonal transforming means for orthogonally transforming output signals of the frequency offset compensating means; and a decoding means for decoding the orthogonally transformed signals.
According to a seventh aspect of the present invention, there is provided a communication apparatus and the above receiving apparatus used in multiplex communication based on multi-carrier modulation where subcarriers of a plurality of channels are cyclically arranged.
According to an eighth aspect of the present invention, there is provided a filter apparatus for extracting a specific signal from a group of multi-carrier modulated signals, including a signal component demultiplexing apparatus comprising branching circuits connected in stages and hierarchically by a branching method, each branching circuit including a symbol delaying means for delaying an input group of signals by N/2(m+1) symbols, a phase offset adjusting means for shifting the phase of the input group of signals by xe2x88x92xcfx80(k/2m) radians with a reference 0 Hz, an adding means for adding an output signal of the symbol delaying means and an output signal of the phase offset adjusting means to calculate one symbol string alternately positioned on a frequency axis in a multiplexed signal input to a signal selecting and outputting means, and a subtracting means for subtracting the output signal of the phase offset adjusting means from the output signal of the symbol delaying means to calculate the other symbol string alternately positioned on the frequency axis in the multiplexed signal input to the signal selecting and outputting means; a signal selecting means for selecting and outputting a group of symbols of a specific subcarrier from among the symbol strings demultiplexed at the signal component demultiplexing apparatus; and a frequency offset compensating means for compensating for frequency offset of the group of symbols selected and output by the signal selecting means.
According to a ninth aspect of the present invention, there is provided a receiving apparatus used in multiplex communication based on multi-carrier modulation where subcarriers of a plurality of channels are cyclically arranged, including a receiving means for receiving a group of multi-carrier modulated signals; the above filter apparatus for extracting a specific signal from the group of multi-carrier modulated signals received at the receiving means; an orthogonal transforming means for orthogonally transforming the signal extracted at the filter apparatus; and a decoding means for decoding the orthogonally transformed signal.
According to a 10th aspect of the present invention, there is provided a filter apparatus for extracting a specific signal from a group of multi-carrier modulated signals, provided with a subcarrier selecting means for selecting a subcarrier, at least one signal selecting means for selecting and outputting a specific group of signals from among the input group of signals in accordance with the selected subcarrier, and a frequency offset compensating means for compensating the frequency offset of a signal selected by the signal selecting means.
According to an 11th aspect of the present invention, there is provided a receiving apparatus used in multiplex communication based on multi-carrier modulation where subcarriers of a plurality of channels are cyclically arranged, provided with a receiving means for receiving a group of multi-carrier modulated signals, the above filter apparatus for extracting a specific signal from the group of multi-carrier modulated signals received at the receiving means, an orthogonal transforming means for orthogonally transforming the signal extracted at the filter apparatus, and a decoding means for decoding the orthogonally transformed signal.
According to a 12th aspect of the present invention, there is provided a filter apparatus for extracting a specific signal from a group of multi-carrier modulated signals, including a pass subcarrier selection signal outputting means for outputting a complex sine wave signal in accordance with a channel to be selected, a multiplying means for multiplying a complex sine wave signal output from the pass subcarrier selection signal outputting means and the input group of signals, at least one signal component demultiplexing means for selecting a specific group of signals from the results of multiplication in the multiplying means, and a symbol rearranging means for rearranging the output of the signal component demultiplexing apparatus on the frequency axis.
According to a 13th aspect of the present invention, there is provided a receiving apparatus used in multiplex communication based on multi-carrier modulation where subcarriers of a plurality of channels are cyclically arranged, including a receiving means for receiving a group of multi-carrier modulated signals, the above filter apparatus for extracting a specific signal from the group of multi-carrier modulated signals received at the receiving means, an orthogonal transforming means for orthogonally transforming the signal extracted at the filter apparatus, and a decoding means for decoding the orthogonally transformed signal.
According to a 14th aspect of the present invention, there is provided a receiving apparatus used in multiplex communication based on multi-carrier modulation where a plurality-of channels of subcarriers are cyclically arranged, provided with a receiving means for receiving a group of multi-carrier modulated signals; a switching means for switching the input group of signals; a buffer means for holding the group of multi-carrier modulated signals received at the receiving means; a filter apparatus connected after the switching means and for selecting and outputting a specific group of signals from the input group of signals; an orthogonal transforming means for orthogonally transforming the signals extracted at the filter apparatus; and a decoding means for decoding the orthogonally transformed signals, wherein the switching means outputs one symbol""s worth of the group of signals to the filter apparatus, and the buffer means holds the input one symbol""s worth of the group of signals during that time and transmits the group of signals held at the buffer means via the switching means to the filter apparatus after the end of transmitting the signals to the filter apparatus and the filter apparatus selects and outputs only a designated subcarrier from the group of signals input via the switching means.
As the filter apparatus, use can be made of the above various filter apparatuses.
According to a 15th aspect of the present invention, there is provided a receiving apparatus used in multiplex communication based on multi-carrier modulation where a plurality of channels of the subcarriers are cyclically arranged, provided with a receiving means for receiving a group of multi-carrier modulated signals; a first filter apparatus for selecting and outputting a group of signals of even number carriers from a group of multi-carrier modulated signals received at the receiving means; a second filter apparatus for selecting and outputting a group of signals of odd number carriers from a group of multi-carrier modulated signals received at the receiving means; a buffer means for holding the output group of signals of the second filter apparatus; a switching means for switching the output group of signals of the first filter apparatus; an orthogonal transforming means connected after the switching means and orthogonally transforming the switched output signals; and a decoding means for decoding the orthogonally transformed signals, wherein the switching means transmits the output signals of the first filter apparatus to the orthogonal transforming means and transmits the group of signals held at the buffer means via the switching means to the orthogonal transforming means after the end of transmitting the signals to the orthogonal transforming means.
As the filter apparatus, use can be made of the above various filter apparatuses.
According to a 16th aspect of the present invention, there is provided a communication method comprising an encoding and transmitting step of independently encoding information of a plurality of channels, arranging signal points by modulating the encoded information based on a predetermined modulation method, multiplexing the plurality of signal point-arranged signals cyclically on a time axis, inversely orthogonally transforming the multiplexed signal, and transmitting the orthogonally transformed information and a receiving and decoding step of receiving the transmitted signal, selecting and outputting only the signal of an intended channel from among the received multiplexed signal after the orthogonal transformation, orthogonally transforming the selected and output signal, and decoding the orthogonally transformed information, wherein the signal selection processing in the receiving step comprises giving a delay of N/2(m+1) symbols, shifting the phase by exactly xcfx80(k/2m) radians, and performing branched and in stages the procedure of adding the symbol delayed signal and the phase shifted signal to calculate one symbol string alternately located on the frequency axis in the input multiplexed signal or subtracting the phase shifted signal from the symbol delayed signal to calculate the other symbol string alternately located on the frequency axis in the input multiplexed signal.